What model receiver are you trying to use?
All DirecTV dishes are not the same and it sounds like you may be trying to move a HD receiver from a home based, SWM system to an RV based non SWM system. So as Grumpy mentioned, unless your using a Winegard Travl'er on the Seneca, you may need to make a few adjustments to get it to work. If your using an older SD receiver like a R22 it should work. The newest H25 or the Genie DVR's will NOT work on a non SWM system, period.
First, any DirecTV HD receiver that is model H24/HR24 or older can work on a non SWM system, but you will need to use a B Band converter on the non SWM system before the receiver will be able to "see" the LNB on the dish.
Also, if your trying to use a DVR from the house, such as an HR21, HR22, etc. you will most likely have to also restrict the receiver in the settings to tell the DVR to only use one tuner and make sure that the B Band converter is on that tuner's coax feed line.